Ultrafast colorimetric humidity sensor and method of preparing the same

ABSTRACT

The present disclosure relates to a colorimetric humidity sensor and a method of preparing the same, and in the colorimetric humidity sensor that is an ultrafast colorimetric humidity sensor including a colorimetric member including humidity-responsive particles configured in a disordered monolayer arrangement on a substrate, the humidity-responsive particles are amorphous, porous, and polydispersed microspheres, and the colorimetric humidity sensor indicates a color change according to humidity upon light irradiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 16/689,123 filed on Nov. 20, 2019, which is a continuationapplication of PCT Application No. PCT/KR2019/014154, filed on Oct. 25,2019, which claims priority to Korean Patent Application Number10-2018-0146381, filed on Nov. 23, 2018. The entire contents of theaforementioned related applications are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to an ultrafast colorimetric humiditysensor and a method of preparing the colorimetric humidity sensor.

BACKGROUND

Possibilities for replacing chemically produced colors with structuralcolors have gained substantial traction in display and sensingapplications, as structural colors do not involve environmentallyhazardous chemicals found in many synthetic pigments and dyes. Popularstrategies for producing structural colors have involved ordered anduniform nano/microstructures using dielectric or plasmonic materials.However, production of these structures generally requires top-downclean-room fabrication methods, which are expensive and time-consuming.Solution-based methods on the other hand are cheap, simple and scalable,but generally produce structures that lack spatial order and exhibit adegree of polydispersity. It is therefore desirable to create structuralcolors from systems that tolerate disorder and non-uniformity, assolution-based methods can open up practical pathways forimplementation. The present inventors have previously demonstrateddisordered structural colors from a random arrangement of polydispersed,crystalline TiO₂ microspheres, synthesized through a simple hydrothermalmethod and annealing step. G. Shang et al. also theoreticallyinvestigated structural colors from randomly arranged monodispersedparticles with optimized geometries for color saturation control. Due tothe ease and cheap costs associated with preparing such systems, a broadrange of opportunities in colorimetric applications based on disorderedstructural colors remains open for exploration. One area of impact thathas not been investigated but holds strong potential is in opticalhumidity sensing.

Humidity sensing represents an indispensable technology for managingproduct quality in a wide range of industries including meteorological,electronics, medicine, food science, and semiconductors. Of the manytypes of humidity sensing platforms, colorimetric sensing provides asimple, visual approach for gauging the relative humidity. Variouscolorimetric systems have been demonstrated with nanostructuresincluding graphene oxide, porphyrin-clay composites, and polymerelectrolytes.

PRIOR ART DOCUMENT

[Non-Patent Document] Shang, G. et al., Photonic Glass for High ContrastStructural Color, Scientific Reports, 2018, 8, 7804

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present disclosure provides an ultrafast colorimetric humiditysensor and a method of preparing the colorimetric humidity sensor thatincludes a colorimetric member including humidity-responsive particlesconfigured in a disordered monolayer arrangement on a substrate, and thehumidity-responsive particles are amorphous, porous, and polydispersedmicrospheres and the colorimetric humidity sensor indicates a colorchange according to humidity upon light irradiation.

However, problems to be solved by the present disclosure are not limitedto the above-described problems. Although not described herein, otherproblems to be solved by the present disclosure can be clearlyunderstood by a person with ordinary skill in the art from the followingdescriptions.

Means for Solving the Problems

A first aspect of the present disclosure provides a colorimetrichumidity sensor including a colorimetric member includinghumidity-responsive particles configured in a disordered monolayerarrangement on a substrate, and the humidity-responsive particles areamorphous, porous, and polydispersed microspheres, and the colorimetrichumidity sensor indicates a color change according to humidity uponlight irradiation.

A second aspect of the present disclosure provides a method of preparinga colorimetric humidity sensor, including: synthesizing amorphousmicrospheres by a non-aqueous solvothermal method to preparehumidity-responsive particles; and coating the humidity-responsiveparticles in a monolayer on a substrate to form a colorimetric member.

Effects of the Invention

According to embodiments of the present disclosure, humidity-responsivestructural colors from a random arrangement of amorphous microspheresare demonstrated. Through a comprehensive suite of characterizationmethods and optical analysis using effective medium theory, it is shownthat a large fraction of the amorphous titania microsphere ismicroporous, permitting significant changes to the effectivepermittivity upon water uptake. Individually the microspheres areindistinguishable by scattered light in dry and humid environments, butthe microspheres display color contrast in the both environments as thesuperposition of the individual scattering spectra, determined by thepolydispersed size distribution and effective permittivity, exposessmoothly varying spectral features unique to the humid environment. Thecorrelation between pore volume filling by water and spectral changeenables the water uptake amount by the microspheres to be attainedoptically using a fraction of material and time required by conventionalgravimetric analysis. Lack of spatial order for color generation alsofacilitates the fabrication of a simple binary humidity-responsivedisplay, optimized in speed and signal at only a monolayer coverage ofparticles. Such results suggest intriguing possibilities for realizingcheap, simple and efficient colorimetric humidity sensors usingstructural colors from disordered systems.

According to embodiments of the present disclosure, reflectivity changesof the material due to moisture-induced volume swelling alters thematerial color, demonstrating the involvement of optical,physicochemical and mechanical processes. Although the combination ofthese mechanisms provides large colorimetric ranges, the swellingprocess limits the response time to a range of hundreds of millisecondsto a few minutes. A colorimetric system that does not involve volumeswelling but only optical and diffusive mechanisms, in principle, couldoffer faster response rates.

According to embodiments of the present disclosure, demonstrated is afast, humidity-sensitive colorimetric system using disordered structuralcolors in the form of randomly arranged amorphous, polydispersed titaniamicrospheres. Although the microspheres individually exhibit a noisyscattering spectrum in the visible range, their ensemble collectivelydisplays a smoothly varying scattering spectrum that translates to anoptimally saturated color. Because scattering from the microspheres isincoherent, the total scattering cross section in the far field can beexpressed as the sum of individual scattering cross sections generatedby every particle. This washes out the spectral noise and exposessmoothly varying features in the total scattering spectrum. Themicrospheres are characterized by a large porosity that, when occupiedby water vapor, changes the effective refractive index of the system.The present inventors exploit the strong correlation between pore volumefilling and spectral change to extract the water uptake amount usingonly a fraction of sample and time required by conventional gravimetricanalysis. The particles of the present disclosure show moderatelyreversible characteristics and rapid response speeds (about 30 ms).Finally, the present disclosure describes, through spin-coating, abinary humidity display optimized in color saturation with only amonolayer coverage of humidity responsive and unresponsive microspheres.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A (i) shows amorphous titania particle set labelled as Arepresenting size distributions as shown in FIG. 1A (ii) with averagediameter of 0.69 μm according to Example of the present disclosure. FIG.1B (i) shows amorphous titania particle set labelled as B representingsize distributions as shown in FIG. 1B (ii) with average diameter of1.03 μm according to Example of the present disclosure. FIG. 1C (i)shows amorphous titania particle set labelled as C representing sizedistributions as shown in FIG. 1C (ii) with average diameter of 1.26 μmaccording to Example of the present disclosure. FIG. 1D (i) showsamorphous titania particle set labelled as D representing sizedistributions as shown in FIG. 1D (ii) with average diameter of 1.45 μmaccording to Example of the present disclosure. FIG. 1E (i) showsamorphous titania particle set labelled as E representing sizedistributions as shown in FIG. 1E (ii) with average diameter of d 1.65μm according to Example of the present disclosure. Herein, FIGS. 1A (i),1B (i), 1C (i), 1D (i), and 1E (i) are FE-SEM images of particles (scalebar: 2 μm), the insets are photographs illustrating forward-scatteredcolors from spin-coated particle sets on glass substrate under dry (top)and ambient (bottom) conditions (scale bar: 1 cm), and FIGS. 1A (ii), 1B(ii), 1C (ii), 1D (ii), and 1E (ii) are measured size distribution ofthe microspheres with 0.08 μm-sized bins and the solid curves representnormal fits to the distribution.

FIG. 1F (i) shows an anatase particle set representing sizedistributions as shown in FIG. 1F (ii) with an average diameter of 0.9μm according to Comparative Example. Herein, FIG. 1F (i) is FE-SEM imageof particles (scale bar: 2 μm), the inset is photographs illustratingforward-scattered colors from spin-coated particle set on glasssubstrate under dry (top) and ambient (bottom) conditions (scale bar: 1cm), and FIG. 1F (ii) is measured size distribution of the microsphereswith 0.08 μm-sized bins and the solid curve represents normal fits tothe distribution.

FIG. 2A shows structural phase from powder XRD of anatase, and amorphousA, B, C, D, and E sets according to Example of the present disclosure.FIG. 2B shows N₂ adsorption-desorption isotherms of anatase, andamorphous A, B, C, D, and E sets according to Example of the presentdisclosure. FIG. 2C shows pore size distribution curves of anatase, andamorphous A, B, C, D, and E sets according to Example of the presentdisclosure.

FIG. 3A shows FT-IR spectra of amorphous D particles in dry and ambientenvironments (about 50% RH) according to Example of the presentdisclosure. FIG. 3B shows FT-IR spectra of amorphous E particles in dryand ambient environments (about 50% RH) according to Example of thepresent disclosure. FIG. 3C shows FT-IR spectra of anatase particles indry and ambient environments (about 50% RH) according to Example of thepresent disclosure.

FIG. 4 is a schematic diagram of light scattering from a polydispersedmicrospheres in dry (left) and saturated humidity (right) environmentsaccording to Example of the present disclosure, and the insets showmodels of pores within amorphous titania filled with air or water vapor.

FIG. 5A shows calculated single particle extinction cross sections, as afunction of wavelength and diameter superimposed with the sizedistribution, shown by the square drawn with white solid lines, and FIG.5B shows measured and calculated total extinction cross sections as afunction of wavelength, for an amorphous D set at 3.3% RH conditionaccording to Example of the present disclosure. FIG. 5C shows calculatedsingle particle extinction cross sections, as a function of wavelengthand diameter superimposed with the size distribution, shown by thesquare drawn with white solid lines, and FIG. 5D shows measured andcalculated total extinction cross sections as a function of wavelength,for an amorphous D set at 97.1% RH condition according to Example of thepresent disclosure.

FIG. 6A shows calculated single particle extinction cross sections, as afunction of wavelength and diameter superimposed with the sizedistribution, shown by the square drawn with white solid lines, and FIG.6B shows measured and calculated total extinction cross sections as afunction of wavelength, for an amorphous E set at 3.3% RH conditionaccording to Example of the present disclosure. FIG. 6C shows calculatedsingle particle extinction cross sections, as a function of wavelengthand diameter superimposed with the size distribution, shown by thesquare drawn with white solid lines, and FIG. 6D shows measured andcalculated total extinction cross sections as a function of wavelength,for an amorphous D set at 97.1% RH condition according to Example of thepresent disclosure.

FIG. 7A shows CIE chromaticity diagram of D set at 3.3% RH and 97.1% RHaccording to Example of the present disclosure. FIG. 7B shows CIEchromaticity diagram of E set at 3.3% RH and 97.1% RH according toExample of the present disclosure.

FIG. 8 shows the measured (top) and fitted (bottom) normalizedextinction cross sections from 3.3% RH to 97.1% RH for the amorphous Dset according to Example of the present disclosure, and λ₀ denotes thespectral position of a reference peak at 580 nm and CIE colors (changedgradually from ivory at RH 3.3% to pink at RH 97.1%) determined with themeasured extinction cross sections from the respective RH values areshown on the uppermost side.

FIG. 9 shows measured peak shift relative to λ₀ for different RH valuesaccording to Example of the present disclosure.

FIG. 10 is a schematic diagram of the pore volume filling model insidethe amorphous titania matrix in different RH conditions according toExample of the present disclosure.

FIG. 11 is a graph showing water uptake percentages from DVSmeasurements (▪) and calculated pore filling fraction (

for different RH values according to Example of the present disclosure.

FIG. 12 shows measured peak shift relative to λ₀ at 3.3% RH to 97.1% RHconditions upon multiple cycles according to Example of the presentdisclosure.

FIG. 13A shows calculated total differential scattering cross sectionsfor the amorphous D set in dry condition according to Example of thepresent disclosure. FIG. 13B shows calculated total differentialscattering cross sections for the amorphous D set in ambient humiditycondition according to Example of the present disclosure.

FIG. 14 is a photograph showing an experimental setup to evaluate thescattered colors as a function of illumination angle θ according toExample of the present disclosure, and the sample and camera are fixedin position while the illumination angle varies.

FIGS. 15A-15R show photographs of the amorphous D set obtained atseveral illumination angles, showing angle-dependent color variation fordry (top, FIGS. 15A-15I) and ambient humidity (bottom, FIGS. 15J-15R))conditions according to Example of the present disclosure.

FIGS. 16A (i)-16A (iii) show photographs of humidity-responsive displaysthat are activated under dry (cactus icon) condition according toExample of the present disclosure, and the right image (FIG. 16A (iii))describes the composition of the icon image and background. FIGS. 16B(i)-16B (iii) show photographs of humidity-responsive displays that areactivated under ambient humidity (rainy cloud icon) condition accordingto Example of the present disclosure, and the right image (FIG. 16B(iii)) describes the composition of the icon image and background.

FIG. 17 is a photograph showing experimental setup to evaluate thescattering behavior of the particles under different sample rotationangles a according to Example of the present disclosure, and theillumination source and camera are optically aligned while the sample isrotated and the rotation angle changes the effective particle density inthe beam path.

FIGS. 18A-18H show photographs of a rainy cloud display rotated to 0°,30°, 45° and 60° under dry (top, FIGS. 18A-18D) and ambient humidity(bottom, FIGS. 18E-18H) conditions according to Example of the presentdisclosure.

FIG. 19 shows a bright field optical image and a photograph (inset) ofthe amorphous D set in dry conditions obtained using a 100×0.9 NAobjective lens according to Example of the present disclosure, and scalebar is 20 μm.

FIG. 20 shows a bright field optical image and a photograph (inset) ofthe amorphous D set in ambient humidity conditions obtained using a100×0.9 NA objective lens according to Example of the presentdisclosure, and scale bar is 20 μm.

FIG. 21A shows FT-IR spectra of the amorphous A set in dry and ambienthumidity conditions according to Example of the present disclosure. FIG.21B shows FT-IR spectra of the amorphous B set in dry and ambienthumidity conditions according to Example of the present disclosure. FIG.21C shows FT-IR spectra of the amorphous C set in dry and ambienthumidity conditions according to Example of the present disclosure.

FIG. 22 is a graph showing the permittivity of anatase TiO₂ andamorphous titania as a function of wavelength according to Example ofthe present disclosure.

FIG. 23A shows calculated single particle extinction cross sections, asa function of wavelength and diameter superimposed with the sizedistribution, shown by the square drawn with white solid lines, and FIG.23B shows measured and calculated total extinction cross sections as afunction of wavelength, for an amorphous A set at 3.3% RH conditionaccording to Example of the present disclosure. FIG. 23C showscalculated single particle extinction cross sections, as a function ofwavelength and diameter superimposed with the size distribution, shownby the square drawn with white solid lines, and FIG. 23D shows measuredand calculated total extinction cross sections as a function ofwavelength, for an amorphous A set at 97.1% RH condition according toExample of the present disclosure.

FIG. 24A shows calculated single particle extinction cross sections, asa function of wavelength and diameter superimposed with the sizedistribution, shown by the square drawn with white solid lines, and FIG.24B shows measured and calculated total extinction cross sections as afunction of wavelength, for an amorphous B set at 3.3% RH conditionaccording to Example of the present disclosure. FIG. 24C showscalculated single particle extinction cross sections, as a function ofwavelength and diameter superimposed with the size distribution, shownby the square drawn with white solid lines, and FIG. 24D shows measuredand calculated total extinction cross sections as a function ofwavelength, for an amorphous B set at 97.1% RH condition according toExample of the present disclosure.

FIG. 25A shows calculated single particle extinction cross sections, asa function of wavelength and diameter superimposed with the sizedistribution, shown by the square drawn with white solid lines, and FIG.25B shows measured and calculated total extinction cross sections as afunction of wavelength, for an amorphous C set at 3.3% RH conditionaccording to Example of the present disclosure. FIG. 25C showscalculated single particle extinction cross sections, as a function ofwavelength and diameter superimposed with the size distribution, shownby the square drawn with white solid lines, and FIG. 25D shows measuredand calculated total extinction cross sections as a function ofwavelength, for an amorphous C set at 97.1% RH condition according toExample of the present disclosure.

FIG. 26A shows CIE chromaticity diagram of A set at 3.3% RH and 97.1% RH(from beige to pink) according to Example of the present disclosure.FIG. 26B shows CIE chromaticity diagram of B set at 3.3% RH and 97.1% RH(from lilac to sky blue) according to Example of the present disclosure.FIG. 26C shows CIE chromaticity diagram of C set at 3.3% RH and 97.1% RH(from sky blue to lemon) according to Example of the present disclosure.

FIG. 27 is a schematic diagram showing time-resolved measurements on ahumidity response according to Example of the present disclosure.

FIG. 28 shows output voltages from a Si photodiode showing changes inscattering at 690 nm by amorphous B particles subjected to differenthumidity modulation frequencies according to Example of the presentdisclosure.

FIG. 29 is a graph showing an evaluation on the rise and recovery timesof amorphous B particles according to Example of the present disclosure.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments and examples will be described in detail withreference to the accompanying drawings so that the present disclosuremay be readily implemented by a person with ordinary skill in the art.However, it is to be noted that the present disclosure is not limited tothe embodiments and examples but can be embodied in various other ways.In the drawings, parts irrelevant to the description are omitted for thesimplicity of explanation, and like reference numerals denote like partsthrough the whole document.

Throughout this document, the term “connected to” may be used todesignate a connection or coupling of one element to another element andincludes both an element being “directly connected to” another elementand an element being “electronically connected to” another element viaanother element.

Through the whole document, the term “on” that is used to designate aposition of one element with respect to another element includes both acase that the one element is adjacent to the other element and a casethat any other element exists between these two elements.

Further, through the whole document, the term “comprises or includes”and/or “comprising or including” used in the document means that one ormore other components, steps, operation and/or existence or addition ofelements are not excluded in addition to the described components,steps, operation and/or elements unless context dictates otherwise.

Through the whole document, the term “about or approximately” or“substantially” is intended to have meanings close to numerical valuesor ranges specified with an allowable error and intended to preventaccurate or absolute numerical values disclosed for understanding of thepresent disclosure from being illegally or unfairly used by anyunconscionable third party.

Through the whole document, the term “step of” does not mean “step for”.

Through the whole document, the term “combination of” included inMarkush type description means mixture or combination of one or morecomponents, steps, operations and/or elements selected from a groupconsisting of components, steps, operation and/or elements described inMarkush type and thereby means that the disclosure includes one or morecomponents, steps, operations and/or elements selected from the Markushgroup.

Through the whole document, a phrase in the form “A and/or B” means “Aor B, or A and B”.

Hereafter, embodiments of the present disclosure will be described indetail, but the present disclosure may not be limited thereto.

A first aspect of the present disclosure provides a colorimetrichumidity sensor including a colorimetric member includinghumidity-responsive particles configured in a disordered monolayerarrangement on a substrate, and the humidity-responsive particles areamorphous, porous, and polydispersed microspheres, and the colorimetrichumidity sensor indicates a color change according to humidity uponlight irradiation.

In an embodiment of the present disclosure, the humidity-responsiveparticles may include at least one oxides, but may not be limitedthereto. In an embodiment of the present disclosure, thehumidity-responsive particles may include at least one oxides selectedfrom SiO₂, TiO₂, BaTiO₃, ZnO, Ta₂O₃, Nb₂O₃, CaO, Li₂O, SnO₂, Sb₂O₃,Sb₂O₄, As₂O₃, SrTiO₃, PbTiO₃, and CaTiO₃, but may not be limitedthereto.

In an embodiment of the present disclosure, the colorimetric member mayindicate a different color according to an average diameter of thehumidity-responsive particles. According to an embodiment of the presentdisclosure, the average diameter of the humidity-responsive particlesmay be from about 0.05 μm to 10 μm, but may not be limited thereto. Forexample, the average diameter of the humidity-responsive particles maybe from about 0.05 μm to about 10 μm, from about 0.05 μm to about 9 μm,from about 0.05 μm to about 8 μm, from about 0.05 μm to about 7 μm, fromabout 0.05 μm to about 6 μm, from about 0.05 μm to about 5 μm, fromabout 0.05 μm to about 4 μm, from about 0.05 μm to about 3 μm, fromabout 0.05 μm to about 2 μm, from about 0.05 μm to about 1 μm, fromabout 0.05 μm to about 0.5 μm, from about 0.05 μm to about 0.2 μm, fromabout 0.05 μm to about 0.1 μm, from about 0.1 μm to about 10 μm, fromabout 0.1 μm to about 9 μm, from about 0.1 μm to about 8 μm, from about0.1 μm to about 7 μm, from about 0.1 μm to about 6 μm, from about 0.1 μmto about 5 μm, from about 0.1 μm to about 4 μm, from about 0.1 μm toabout 3 μm, from about 0.1 μm to about 2 μm, from about 0.1 μm to about1 μm, from about 0.1 μm to about 0.5 μm, from about 0.1 μm to about 0.2μm, from about 0.2 μm to about 10 μm, from about 0.2 μm to about 9 μm,from about 0.2 μm to about 8 μm, from about 0.2 μm to about 7 μm, fromabout 0.2 μm to about 6 μm, from about 0.2 μm to about 5 μm, from about0.2 μm to about 4 μm, from about 0.2 μm to about 3 μm, from about 0.2 μmto about 2 μm, from about 0.2 μm to about 1 μm, from about 0.2 μm toabout 0.5 μm, from about 0.5 μm to about 10 μm, from about 0.5 μm toabout 9 μm, from about 0.5 μm to about 8 μm, from about 0.5 μm to about7 μm, from about 0.5 μm to about 6 μm, from about 0.5 μm to about 5 μm,from about 0.5 μm to about 4 μm, from about 0.5 μm to about 3 μm, fromabout 0.5 μm to about 2 μm, from about 0.5 μm to about 1 μm, from about1 μm to about 10 μm, from about 1 μm to about 9 μm, from about 1 μm toabout 8 μm, from about 1 μm to about 7 μm, from about 1 μm to about 6μm, from about 1 μm to about 5 μm, from about 1 μm to about 4 μm, fromabout 1 μm to about 3 μm, from about 1 μm to about 2 μm, from about 2 μmto about 10 μm, from about 2 μm to about 9 μm, from about 2 μm to about8 μm, from about 2 μm to about 7 μm, from about 2 μm to about 6 μm, fromabout 2 μm to about 5 μm, from about 2 μm to about 4 μm, from about 2 μmto about 3 μm, from about 3 μm to about 10 μm, from about 3 μm to about9 μm, from about 3 μm to about 8 μm, from about 3 μm to about 7 μm, fromabout 3 μm to about 6 μm, from about 3 μm to about 5 μm, from about 3 μmto about 4 μm, from about 4 μm to about 10 μm, from about 4 μm to about9 μm, from about 4 μm to about 8 μm, from about 4 μm to about 7 μm, fromabout 4 μm to about 6 μm, from about 4 μm to about 5 μm, from about 5 μmto about 10 μm, from about 5 μm to about 9 μm, from about 5 μm to about8 μm, from about 5 μm to about 7 μm, from about 5 μm to about 6 μm, fromabout 6 μm to about 10 μm, from about 6 μm to about 9 μm, from about 6μm to about 8 μm, from about 6 μm to about 7 μm, from about 7 μm toabout 10 μm, from about 7 μm to about 9 μm, from about 7 μm to about 8μm, from about 8 μm to about 10 μm, from about 8 μm to about 9 μm, orfrom about 9 μm to about 10 μm, but may not be limited thereto.

Further, in an embodiment of the present disclosure, thehumidity-responsive particles may have a size distribution of from about0.2 μm to about 2 μm, but may not be limited thereto. For example, thehumidity-responsive particles may have a size distribution of from about0.2 μm to about 2 μm, from about 0.2 μm to about 1.8 μm, from about 0.2μm to about 1.6 μm, from about 0.2 μm to about 1.4 μm, from about 0.2 μmto about 1.2 μm, from about 0.2 μm to about 1 μm, from about 0.2 μm toabout 0.8 μm, from about 0.2 μm to about 0.6 μm, from about 0.2 μm toabout 0.4 μm, from about 0.4 μm to about 2 μm, from about 0.6 μm toabout 2 μm, from about 0.8 μm to about 2 μm, from about 1 μm to about 2μm, from about 1.2 μm to about 2 μm, from about 1.4 μm to about 2 μm,from about 1.6 μm to about 2 μm, or from about 1.8 μm to about 2 μm, butmay not be limited thereto.

In an embodiment of the present disclosure, an average size of pores inthe humidity-responsive particles may be from about 1 nm to about 60 nm,but may not be limited thereto. For example, the average size of thepores in the humidity-responsive particles may be from about 1 nm toabout 60 nm, from about 1 nm to about 50 nm, from about 1 nm to about 40nm, from about 1 nm to about 30 nm, from about 1 nm to about 20 nm, fromabout 1 nm to about 10 nm, from about 1 nm to about 5 nm, from about 5nm to about 60 nm, from about 10 nm to about 60 nm, from about 20 nm toabout 60 nm, from about 30 nm to about 60 nm, from about 40 nm to about60 nm, or from about 50 nm to about 60 nm, but may not be limitedthereto.

In an embodiment of the present disclosure, the disordered monolayerarrangement of the humidity-responsive particles formed on the substratemay be formed by arranging the amorphous and porous microspheres into amonolayer having a polydispersed size distribution ranging from about0.05 μm to about 10 μm or from about 0.2 μm to about 2 μm.

In an embodiment of the present disclosure, the humidity-responsiveparticles have pores, and a porosity of the humidity-responsiveparticles may be from about 20% to about 70%, but may not be limitedthereto. For example, the porosity of the humidity-responsive particlesmay be from about 20% to about 70%, from about 20% to about 60%, fromabout 20% to about 50%, from about 20% to about 40%, from about 20% toabout 30%, from about 30% to about 70%, from about 40% to about 70%,from about 50% to about 70%, or from about 60% to about 70%, but may notbe limited thereto.

In an embodiment of the present disclosure, the colorimetric humiditysensor shows a faster response than a conventional humidity sensor, andfor example, a response time of the colorimetric humidity sensor may befrom about 0.1 μs to about 500 ms, but may not be limited thereto. Forexample, the response time of the colorimetric humidity sensor may befrom about 0.1 μs to about 500 ms, from about 0.1 μs to about 400 ms,from about 0.1 μs to about 300 ms, from about 0.1 μs to about 200 ms,from about 0.1 μs to about 100 ms, from about 0.1 μs to about 10 ms,from about 0.1 μs to about 1 ms, from about 0.1 μs to about 500 μs, fromabout 0.1 μs to about 100 μs, from about 0.1 μs to about 10 μs, fromabout 0.1 μs to about 1 μs, from about 1 μs to about 500 ms, from about10 μs to about 500 ms, from about 100 μs to about 500 ms, from about 500μs to about 500 ms, from about 1 ms to about 500 ms, from about 10 ms toabout 500 ms, from about 10 ms to about 50 ms, from about 100 ms toabout 500 ms, from about 200 ms to about 500 ms, from about 300 ms toabout 500 ms, or from about 400 ms to about 500 ms, but may not belimited thereto.

In an embodiment of the present disclosure, the colorimetric humiditysensor may further include humidity-unresponsive particles configured ina monolayer arrangement, but may not be limited thereto. Thehumidity-unresponsive particles are crystalline particles andsubstantially nonporous. For example, the humidity-unresponsiveparticles do not have pores having a size of about 50 nm or lesstherein.

In an embodiment of the present disclosure, the humidity-unresponsiveparticles may include oxides and may include at least oxides selectedfrom SiO₂, TiO₂, BaTiO₃, ZnO, Ta₂O₃, Nb₂O₃, CaO, Li₂O, SnO₂, Sb₂O₃,Sb₂O₄, As₂O₃, SrTiO₃, PbTiO₃, and CaTiO₃, but may not be limitedthereto. For example, the crystalline particles may include TiO₂particles having a crystal structure such as an anatase- or rutile-type,but may not be limited thereto.

In an embodiment of the present disclosure, an average diameter of thehumidity-unresponsive particles may be from about 0.05 μm to about 10μm, but may not be limited thereto. For example, the average diameter ofthe humidity-unresponsive particles may be from about 0.05 μm to about10 μm, from about 0.05 μm to about 9 μm, from about 0.05 μm to about 8μm, from about 0.05 μm to about 7 μm, from about 0.05 μm to about 6 μm,from about 0.05 μm to about 5 μm, from about 0.05 μm to about 4 μm, fromabout 0.05 μm to about 3 μm, from about 0.05 μm to about 2 μm, fromabout 0.05 μm to about 1 μm, from about 0.05 μm to about 0.5 μm, fromabout 0.05 μm to about 0.2 μm, from about 0.05 μm to about 0.1 μm, fromabout 0.1 μm to about 10 μm, from about 0.1 μm to about 9 μm, from about0.1 μm to about 8 μm, from about 0.1 μm to about 7 μm, from about 0.1 μmto about 6 μm, from about 0.1 μm to about 5 μm, from about 0.1 μm toabout 4 μm, from about 0.1 μm to about 3 μm, from about 0.1 μm to about2 μm, from about 0.1 μm to about 1 μm, from about 0.1 μm to about 0.5μm, from about 0.1 μm to about 0.2 μm, from about 0.2 μm to about 10 μm,from about 0.2 μm to about 9 μm, from about 0.2 μm to about 8 μm, fromabout 0.2 μm to about 7 μm, from about 0.2 μm to about 6 μm, from about0.2 μm to about 5 μm, from about 0.2 μm to about 4 μm, from about 0.2 μmto about 3 μm, from about 0.2 μm to about 2 μm, from about 0.2 μm toabout 1 μm, from about 0.2 μm to about 0.5 μm, from about 0.5 μm toabout 10 μm, from about 0.5 μm to about 9 μm, from about 0.5 μm to about8 μm, from about 0.5 μm to about 7 μm, from about 0.5 μm to about 6 μm,from about 0.5 μm to about 5 μm, from about 0.5 μm to about 4 μm, fromabout 0.5 μm to about 3 μm, from about 0.5 μm to about 2 μm, from about0.5 μm to about 1 μm, from about 1 μm to about 10 μm, from about 1 μm toabout 9 μm, from about 1 μm to about 8 μm, from about 1 μm to about 7μm, from about 1 μm to about 6 μm, from about 1 μm to about 5 μm, fromabout 1 μm to about 4 μm, from about 1 μm to about 3 μm, from about 1 μmto about 2 μm, from about 2 μm to about 10 μm, from about 2 μm to about9 μm, from about 2 μm to about 8 μm, from about 2 μm to about 7 μm, fromabout 2 μm to about 6 μm, from about 2 μm to about 5 μm, from about 2 μmto about 4 μm, from about 2 μm to about 3 μm, from about 3 μm to about10 μm, from about 3 μm to about 9 μm, from about 3 μm to about 8 μm,from about 3 μm to about 7 μm, from about 3 μm to about 6 μm, from about3 μm to about 5 μm, from about 3 μm to about 4 μm, from about 4 μm toabout 10 μm, from about 4 μm to about 9 μm, from about 4 μm to about 8μm, from about 4 μm to about 7 μm, from about 4 μm to about 6 μm, fromabout 4 μm to about 5 μm, from abut 5 μm to about 10 μm, from about 5 μmto about 9 μm, from about 5 μm to about 8 μm, from about 5 μm to about 7μm, from about 5 μm to about 6 μm, from about 6 μm to about 10 μm, fromabout 6 μm to about 9 μm, from about 6 μm to about 8 μm, from about 6 μmto about 7 μm, from about 7 μm to about 10 μm, from about 7 μm to about9 μm, from about 7 μm to about 8 μm, from about 8 μm to about 10 μm,from about 8 μm to about 9 μm, or from about 9 μm to about 10 μm, butmay not be limited thereto.

Further, in an embodiment of the present disclosure, thehumidity-unresponsive particles may have an average diameter of fromabout 0.2 μm to about 2 μm, but may not be limited thereto. For example,the humidity-unresponsive particles may have an average diameter of fromabout 0.2 μm to about 2 μm, from about 0.2 μm to about 1.8 μm, fromabout 0.2 μm to about 1.6 μm, from about 0.2 μm to about 1.4 μm, fromabout 0.2 μm to about 1.2 μm, from about 0.2 μm to about 1 μm, fromabout 0.2 μm to about 0.8 μm, from about 0.2 μm to about 0.6 μm, fromabout 0.2 μm to about 0.4 μm, from about 0.4 μm to about 2 μm, fromabout 0.6 μm to about 2 μm, from about 0.8 μm to about 2 μm, from about1 μm to about 2 μm, from about 1.2 μm to about 2 μm, from about 1.4 μmto about 2 μm, from about 1.6 μm to about 2 μm, or from about 1.8 μm toabout 2 μm, but may not be limited thereto.

In another embodiment of the present disclosure, the colorimetrichumidity sensor may further include a second colorimetric memberincluding second humidity-responsive particles configured in adisordered monolayer arrangement, and an average diameter of the secondhumidity-responsive particles is different from that of thehumidity-responsive particles, but may not be limited thereto. Forexample, the second colorimetric member including the secondhumidity-responsive particles configured in a disordered monolayerarrangement may be arranged on the colorimetric member, on the monolayerarrangement of the humidity-unresponsive particles in the colorimetricmember, or between the colorimetric member and the monolayer arrangementof the humidity-unresponsive particles. For example, the secondcolorimetric member including the second humidity-responsive particlesconfigured in a disordered monolayer arrangement is formed to haveslight or less color change according to humidity than the colorimetricmember. As a non-limiting example, if a display icon is configured usingthe humidity-responsive particles and the background except the displayicon is configured using the humidity-unresponsive particles and/or thesecond humidity-responsive particles, only the humidity-responsiveparticles may change in color according to humidity and the color of theparticles forming the background does not change or change slightly, andthus, only the color of the display icon can be changed remarkably asthe humidity is changed.

In an embodiment of the present disclosure, an average diameter of thesecond humidity-responsive particles is different from that of thehumidity-responsive particles and may be from about 0.05 μm to about 10μm, but may not be limited thereto. For example, the average diameter ofthe second humidity-responsive particles may be from about 0.05 μm toabout 10 μm, from about 0.05 μm to about 9 μm, from about 0.05 μm toabout 8 μm, from about 0.05 μm to about 7 μm, from about 0.05 μm toabout 6 μm, from about 0.05 μm to about 5 μm, from about 0.05 μm toabout 4 μm, from about 0.05 μm to about 3 μm, from about 0.05 μm toabout 2 μm, from about 0.05 μm to about 1 μm, from about 0.05 μm toabout 0.5 μm, from about 0.05 μm to about 0.2 μm, from about 0.05 μm toabout 0.1 μm, from about 0.1 μm to about 10 μm, from about 0.1 μm toabout 9 μm, from about 0.1 μm to about 8 μm, from about 0.1 μm to about7 μm, from about 0.1 μm to about 6 μm, from about 0.1 μm to about 5 μm,from about 0.1 μm to about 4 μm, from about 0.1 μm to about 3 μm, fromabout 0.1 μm to about 2 μm, from about 0.1 μm to about 1 μm, from about0.1 μm to about 0.5 μm, from about 0.1 μm to about 0.2 μm, from about0.2 μm to about 10 μm, from about 0.2 μm to about 9 μm, from about 0.2μm to about 8 μm, from about 0.2 μm to about 7 μm, from about 0.2 μm toabout 6 μm, from about 0.2 μm to about 5 μm, from about 0.2 μm to about4 μm, from about 0.2 μm to about 3 μm, from about 0.2 μm to about 2 μm,from about 0.2 μm to about 1 μm, from about 0.2 μm to about 0.5 μm, fromabout 0.5 μm to about 10 μm, from about 0.5 μm to about 9 μm, from about0.5 μm to about 8 μm, from about 0.5 μm to about 7 μm, from about 0.5 μmto about 6 μm, from about 0.5 μm to about 5 μm, from about 0.5 μm toabout 4 μm, from about 0.5 μm to about 3 μm, from about 0.5 μm to about2 μm, from about 0.5 μm to about 1 μm, from about 1 μm to about 10 μm,from about 1 μm to about 9 μm, from about 1 μm to about 8 μm, from about1 μm to about 7 μm, from about 1 μm to about 6 μm, from about 1 μm toabout 5 μm, from about 1 μm to about 4 μm, from about 1 μm to about 3μm, from about 1 μm to about 2 μm, from about 2 μm to about 10 μm, fromabout 2 μm to about 9 μm, from about 2 μm to about 8 μm, from about 2 μmto about 7 μm, from about 2 μm to about 6 μm, from about 2 μm to about 5μm, from about 2 μm to about 4 μm, from about 2 μm to about 3 μm, fromabout 3 μm to about 10 μm, from about 3 μm to about 9 μm, from about 3μm to about 8 μm, from about 3 μm to about 7 μm, from about 3 μm toabout 6 μm, from about 3 μm to about 5 μm, from about 3 μm to about 4μm, from about 4 μm to about 10 μm, from about 4 μm to about 9 μm, fromabout 4 μm to about 8 μm, from about 4 μm to about 7 μm, from about 4 μmto about 6 μm, from about 4 μm to about 5 μm, from about 5 μm to about10 μm, from about 5 μm to about 9 μm, from about 5 μm to about 8 μm,from about 5 μm to about 7 μm, from about 5 μm to about 6 μm, from about6 μm to about 10 μm, from about 6 μm to about 9 μm, from about 6 μm toabout 8 μm, from about 6 μm to about 7 μm, from about 7 μm to about 10μm, from about 7 μm to about 9 μm, from about 7 μm to about 8 μm, fromabout 8 μm to about 10 μm, from about 8 μm to about 9 μm, or from about9 μm to about 10 μm, but may not be limited thereto.

Further, in an embodiment of the present disclosure, the averagediameter of the second humidity-responsive particles may be, e.g., fromabout 0.2 μm to about 2 μm, from about 0.2 μm to about 1.8 μm, fromabout 0.2 μm to about 1.6 μm, from about 0.2 μm to about 1.4 μm, fromabout 0.2 μm to about 1.2 μm, from about 0.2 μm to about 1 μm, fromabout 0.2 μm to about 0.8 μm, from about 0.2 μm to about 0.6 μm, fromabout 0.2 μm to about 0.4 μm, from about 0.4 μm to about 2 μm, fromabout 0.6 μm to about 2 μm, from about 0.8 μm to about 2 μm, from about1 μm to about 2 μm, from about 1.2 μm to about 2 μm, from about 1.4 μmto about 2 μm, from about 1.6 μm to about 2 μm, or from about 1.8 μm toabout 2 μm, but may not be limited thereto.

In an embodiment of the present disclosure, moisture may be adsorbed inthe pores of the humidity-responsive particles. Also, in an embodimentof the present disclosure, the colorimetric humidity sensor may indicatea change in saturation of the color according to an angle of lightirradiation.

In an embodiment of the present disclosure, the humidity-responsiveparticles may express structural colors responsive to the humidity inthe disordered arrangement. The microspheres according to an embodimentof the present disclosure have a high porosity and thus can absorbmoisture into the pores. Thus, when moisture is absorbed, thehumidity-responsive particles significantly change in effectivepermittivity.

In an embodiment of the present disclosure, the color change of thecolorimetric humidity sensor upon light irradiation can be measured by atypical optical measuring instrument, but the present disclosure may notbe limited thereto. In an embodiment of the present disclosure, thecolor change of the colorimetric humidity sensor upon light irradiationmay be measured by an optical measuring instrument using at least oneselected from a photodiode, a charge coupled device (CCD), and acomplementary metal oxide semiconductor (CMOS), but may not be limitedthereto.

A second aspect of the present disclosure provides a method of preparinga colorimetric humidity sensor, including: synthesizing amorphousmicrospheres by a non-aqueous solvothermal method to preparehumidity-responsive particles; and coating the humidity-responsiveparticles in a monolayer on a substrate to form a colorimetric member.

All the descriptions of the colorimetric humidity sensor in accordancewith the first aspect of the present disclosure can be applied to themethod of preparing a colorimetric humidity sensor in accordance withthe second aspect of the present disclosure. Detailed descriptions ofparts of the second aspect, which overlap with those of the firstaspect, are omitted hereinafter, but the descriptions of the firstaspect of the present disclosure may be identically applied to thesecond aspect of the present disclosure, even though they are omittedhereinafter.

In an embodiment of the present disclosure, the humidity-responsiveparticles may have amorphous, porous, and polydispersed nature.

In an embodiment of the present disclosure, the colorimetric member mayinclude the humidity-responsive particles configured in a disorderedmonolayer arrangement.

In an embodiment of the present disclosure, the amorphous microspheresmay be synthesized to have different average diameters, respectively,but may not be limited thereto.

In an embodiment of the present disclosure, the humidity-responsiveparticles may include at least one oxides, but may not be limitedthereto. In an embodiment of the present disclosure, thehumidity-responsive particles may include at least one oxides selectedfrom SiO₂, TiO₂, BaTiO₃, ZnO, Ta₂O₃, Nb₂O₃, CaO, Li₂O, SnO₂, Sb₂O₃,Sb₂O₄, As₂O₃, SrTiO₃, PbTiO₃, and CaTiO₃, but may not be limitedthereto.

In an embodiment of the present disclosure, the colorimetric member mayindicate a different color according to an average diameter of thehumidity-responsive particles. According to an embodiment of the presentdisclosure, the average diameter of the humidity-responsive particlesmay be from about 0.05 μm to 10 μm, but may not be limited thereto. Forexample, the average diameter of the humidity-responsive particles maybe from about 0.05 μm to about 10 μm, from about 0.05 μm to about 9 μm,from about 0.05 μm to about 8 μm, from about 0.05 μm to about 7 μm, fromabout 0.05 μm to about 6 μm, from about 0.05 μm to about 5 μm, fromabout 0.05 μm to about 4 μm, from about 0.05 μm to about 3 μm, fromabout 0.05 μm to about 2 μm, from about 0.05 μm to about 1 μm, fromabout 0.05 μm to about 0.5 μm, from about 0.05 μm to about 0.2 μm, fromabout 0.05 μm to about 0.1 μm, from about 0.1 μm to about 10 μm, fromabout 0.1 μm to about 9 μm, from about 0.1 μm to about 8 μm, from about0.1 μm to about 7 μm, from about 0.1 μm to about 6 μm, from about 0.1 μmto about 5 μm, from about 0.1 μm to about 4 μm, from about 0.1 μm toabout 3 μm, from about 0.1 μm to about 2 μm, from about 0.1 μm to about1 μm, from about 0.1 μm to about 0.5 μm, from about 0.1 μm to about 0.2μm, from about 0.2 μm to about 10 μm, from about 0.2 μm to about 9 μm,from about 0.2 μm to about 8 μm, from about 0.2 μm to about 7 μm, fromabout 0.2 μm to about 6 μm, from about 0.2 μm to about 5 μm, from about0.2 μm to about 4 μm, from about 0.2 μm to about 3 μm, from about 0.2 μmto about 2 μm, from about 0.2 μm to about 1 μm, from about 0.2 μm toabout 0.5 μm, from about 0.5 μm to about 10 μm, from about 0.5 μm toabout 9 μm, from about 0.5 μm to about 8 μm, from about 0.5 μm to about7 μm, from about 0.5 μm to about 6 μm, from about 0.5 μm to about 5 μm,from about 0.5 μm to about 4 μm, from about 0.5 μm to about 3 μm, fromabout 0.5 μm to about 2 μm, from about 0.5 μm to about 1 μm, from about1 μm to about 10 μm, from about 1 μm to about 9 μm, from about 1 μm toabout 8 μm, from about 1 μm to about 7 μm, from about 1 μm to about 6μm, from about 1 μm to about 5 μm, from about 1 μm to about 4 μm, fromabout 1 μm to about 3 μm, from about 1 μm to about 2 μm, from about 2 μmto about 10 μm, from about 2 μm to about 9 μm, from about 2 μm to about8 μm, from about 2 μm to about 7 μm, from about 2 μm to about 6 μm, fromabout 2 μm to about 5 μm, from about 2 μm to about 4 μm, from about 2 μmto about 3 μm, from about 3 μm to about 10 μm, from about 3 μm to about9 μm, from about 3 μm to about 8 μm, from about 3 μm to about 7 μm, fromabout 3 μm to about 6 μm, from about 3 μm to about 5 μm, from about 3 μmto about 4 μm, from about 4 μm to about 10 μm, from about 4 μm to about9 μm, from about 4 μm to about 8 μm, from about 4 μm to about 7 μm, fromabout 4 μm to about 6 μm, from about 4 μm to about 5 μm, from about 5 μmto about 10 μm, from about 5 μm to about 9 μm, from about 5 μm to about8 μm, from about 5 μm to about 7 μm, from about 5 μm to about 6 μm, fromabout 6 μm to about 10 μm, from about 6 μm to about 9 μm, from about 6μm to about 8 μm, from about 6 μm to about 7 μm, from about 7 μm toabout 10 μm, from about 7 μm to about 9 μm, from about 7 μm to about 8μm, from about 8 μm to about 10 μm, from about 8 μm to about 9 μm, orfrom about 9 μm to about 10 μm, but may not be limited thereto.

Further, in an embodiment of the present disclosure, thehumidity-responsive particles may have a size distribution of from about0.2 μm to about 2 μm, but may not be limited thereto. For example, thehumidity-responsive particles may have a size distribution of from about0.2 μm to about 2 μm, from about 0.2 μm to about 1.8 μm, from about 0.2μm to about 1.6 μm, from about 0.2 μm to about 1.4 μm, from about 0.2 μmto about 1.2 μm, from about 0.2 μm to about 1 μm, from about 0.2 μm toabout 0.8 μm, from about 0.2 μm to about 0.6 μm, from about 0.2 μm toabout 0.4 μm, from about 0.4 μm to about 2 μm, from about 0.6 μm toabout 2 μm, from about 0.8 μm to about 2 μm, from about 1 μm to about 2μm, from about 1.2 μm to about 2 μm, from about 1.4 μm to about 2 μm,from about 1.6 μm to about 2 μm, or from about 1.8 μm to about 2 μm, butmay not be limited thereto.

In an embodiment of the present disclosure, an average size of pores inthe microspheres may be from about 1 nm to about 60 nm, but may not belimited thereto. For example, the average size of the pores in themicrospheres may be from about 1 nm to about 60 nm, from about 1 nm toabout 50 nm, from about 1 nm to about 40 nm, from about 1 nm to about 30nm, from about 1 nm to about 20 nm, from about 1 nm to about 10 nm, fromabout 1 nm to about 5 nm, from about 5 nm to about 60 nm, from about 10nm to about 60 nm, from about 20 nm to about 60 nm, from about 30 nm toabout 60 nm, from about 40 nm to about 60 nm, or from about 50 nm toabout 60 nm, but may not be limited thereto.

In an embodiment of the present disclosure, the humidity-responsiveparticles have pores, and a porosity of the humidity-responsiveparticles may be from about 20% to about 70%, but may not be limitedthereto. For example, the porosity of the humidity-responsive particlesmay be from about 20% to about 70%, from about 20% to about 60%, fromabout 20% to about 50%, from about 20% to about 40%, from about 20% toabout 30%, from about 30% to about 70%, from about 40% to about 70%,from about 50% to about 70%, or from about 60% to about 70%, but may notbe limited thereto.

In an embodiment of the present disclosure, the forming of thecolorimetric member may be performed through spin-coating, but may notbe limited thereto. Further, in an embodiment of the present disclosure,the colorimetric member may include the amorphous microspheresconfigured in a disordered monolayer arrangement.

In an embodiment of the present disclosure, second humidity-responsiveparticles may be prepared by synthesizing the amorphous microsphereshaving different average diameters, respectively, but may not be limitedthereto. When the humidity-responsive particles are prepared, an averagediameter of the amorphous microspheres can be controlled.

In an embodiment of the present disclosure, the method of preparing acolorimetric humidity sensor may further include coatinghumidity-unresponsive particles or the second humidity-responsiveparticles in a monolayer on the substrate, but may not be limitedthereto. The monolayer of the humidity-unresponsive particles or thesecond humidity-responsive particles may be formed in a region of thesubstrate where the colorimetric member is not formed.

In an embodiment of the present disclosure, in the colorimetric member,the amorphous microspheres, the humidity-responsive particles, thehumidity-unresponsive particles or the second humidity-responsiveparticles may be configured in a disordered monolayer arrangement, butmay not be limited thereto.

In an embodiment of the present disclosure, the humidity-unresponsiveparticles are crystalline particles and substantially nonporous. Forexample, the humidity-unresponsive particles do not have pores having asize of about 50 nm or less therein.

In an embodiment of the present disclosure, the humidity-unresponsiveparticles may include oxides and may include at least one oxidesselected from SiO₂, TiO₂, BaTiO₃, ZnO, Ta₂O₃, Nb₂O₃, CaO, Li₂O, SnO₂,Sb₂O₃, Sb₂O₄, As₂O₃, SrTiO₃, PbTiO₃, and CaTiO₃, but may not be limitedthereto. For example, the crystalline particles may include TiO₂particles having a crystal structure such as an anatase- or rutile-type,but may not be limited thereto.

In an embodiment of the present disclosure, humidity-responsivestructural colors from a random arrangement of amorphous microspheresare demonstrated. Through a comprehensive suite of characterizationmethods and optical analysis using effective medium theory, it is shownthat a large fraction of the amorphous titania microsphere ismicroporous, permitting significant changes to the effectivepermittivity upon water uptake. Individually the microspheres areindistinguishable by scattered light in dry and humid environments, butthe microspheres display color contrast in the both environments as thesuperposition of the individual scattering spectra, determined by thepolydispersed size distribution and effective permittivity, exposessmoothly varying spectral features unique to the humid environment. Thecorrelation between pore volume filling by water and spectral changeenables the water uptake amount by the microspheres to be attainedoptically using a fraction of material and time required by conventionalgravimetric analysis. Lack of spatial order for color generation alsofacilitates the fabrication of a simple binary humidity-responsivedisplay, optimized in speed and signal at only a monolayer coverage ofparticles. Such results suggest intriguing possibilities for realizingcheap, simple and efficient colorimetric humidity sensors usingstructural colors from disordered systems.

In an embodiment of the present disclosure, reflectivity changes of thematerial due to moisture-induced volume swelling alters the materialcolor, demonstrating the involvement of optical, physiochemical andmechanical processes. Although the combination of these mechanismsprovides large colorimetric ranges, the swelling process limits theresponse time to a range of hundreds of milliseconds to a few minutes. Acolorimetric system that does not involve volume swelling but onlyoptical and diffusive mechanisms, in principle, could offer fasterresponse rates.

In an embodiment of the present disclosure, demonstrated is a fast,humidity-sensitive colorimetric system using disordered structuralcolors in the form of randomly arranged amorphous, polydispersed titaniamicrospheres. Although the microspheres individually exhibit a noisyscattering spectrum in the visible range, their ensemble collectivelydisplays a smoothly varying scattering spectrum that translates to anoptimally saturated color. Because scattering from the microspheres isincoherent, the total scattering cross section in the far field can beexpressed as the sum of individual scattering cross sections generatedby every particle. This washes out the spectral noise and exposessmoothly varying features in the total scattering spectrum. Themicrospheres are characterized by a large porosity that, when occupiedby water vapor, changes the effective refractive index of the system. Inthe present disclosure, the strong correlation between pore volumefilling and spectral change is exploited to extract the water uptakeamount using only a fraction of sample and time required by conventionalgravimetric analysis. The particles of the present disclosure showmoderately reversible characteristics and rapid responses. Finally, thepresent disclosure describes, through spin-coating, a binary humiditydisplay optimized in color saturation with only a monolayer coverage ofhumidity responsive and unresponsive microspheres.

Hereinafter, the present disclosure will be explained in more detailwith reference to Examples. However, the following Examples areillustrative only for better understanding of the present disclosure butdo not limit the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present disclosure will be described in more detailwith reference to Examples, but the present disclosure may not belimited thereto.

EXAMPLES

Microsphere Synthesis

Submicron sized titania microspheres were synthesized via a non-aqueoussolvothermal process using a standard procedure as reported previously,which follows the Ostwald ripening mechanism in providingsize-controllable and smooth microspheres. Typically, 5 ml acetylacetonewas dissolved in a 20 ml solvent mixture of Isopropyl alcohol andacetone. Then, tetrabutyl orthotitanate (TBOT) (from 4 mmol to 6 mmol)was dropped rapidly into the solution, followed by the final addition ofethylene glycol. The solution was mixed under vigorous stirring for 20min at ambient temperature. The homogenous transparent yellowish mixturewas transferred to a 50 ml Teflon-lined stainless-steel autoclave, andthen placed inside a muffle furnace for thermal treatment at 200° C. for3 hr. The yellowish precipitate was then collected by centrifugation,washed with acetone and ethanol for several times and dried at 60° C.For use as Comparative Example, TiO₂ anatase microspheres were preparedby further annealing the amorphous titania microspheres at 500° C. for 3hr.

Characterization of the Microspheres

The morphology of the microspheres was analyzed by a Field EmissionScanning Electron Microscope (FESEM, JEOL, JSM-6700F) at an acceleratingvoltage of 10 kV. The powder X-Ray diffraction (XRD) patterns werecollected on a Rigaku D/Max-2000/PC diffractometer with Cu Kα radiation(λ=1.5418 Å) at 25° C. with a tube accelerating voltage and appliedcurrent of 40 kV and 30 mA, respectively. The porosity analysis wasmonitored with a Micromeritics volumetric adsorption analyzer (BELSORPmini II) at 77 K. Before the measurements, the samples were pre-treatedunder vacuum at 125° C. for 5 hr. The functional groups of the sampleswere investigated using a Varian FTS-800 Scimitar series infraredspectrometer in a potassium bromide (KBr) matrix over an energy range offrom 4,000 cm⁻¹ to 400 cm⁻¹.

Extinction Measurements at Different Relative Humidity (RH) environments

Saturated-salt solutions, composed of salt slurries made with distilledwater, were used for controlling the RH. The RH values were calibratedthrough a hygrometer. The measured RH values of the silica gel andcorresponding saturated salts solutions were as follows: silica gel(3.3%), KOH (8.0%), CH₃COOK (22.3%), MgCl₂ (34.9%), K₂CO₃ (45.8%),Mg(NO₃)₂ (53.5%), NaCl (75.7%), and K₂SO₄ (97.1%). Extinctionmeasurements were carried out with a UV-Vis spectrophotometer (UV-vis,SHIMADZU, UV-2450) on microsphere samples that were dispersed onto aglass slide and enclosed in a sealed cuvette cell containing thesaturated-salt solution.

Gravimetric Analysis

Dynamic vapor sorption (surface measurement systems, DVS advantage) wasperformed to measure the amount of water uptake as a function of RH. TheRH was incrementally increased at room temperature from 0% to 100% at aninterval of 5% by changing the ratio of the gas mixture of dry andsaturated gases. Amorphous titania microspheres (set D, 13.8 mg) wereused for the analysis. The particles were preheated at 150° C. for 3 hrto fully remove adsorbed water molecules. The sample mass was measuredevery 1 min throughout the RH scan.

Calculation of Differential Scattering Cross Section

The angle dependent scattering was analytically calculated using Mietheory. The explicit expressions for the scattering coefficients a_(n)and b_(n) for a single particle are given by:

$\begin{matrix}{a_{n} = \frac{{\varepsilon{{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}\left\lbrack {ka{j_{n}\left( {ka} \right)}} \right\rbrack}^{\prime}} - {{j_{n}\left( {ka} \right)}\left\lbrack {\sqrt{\varepsilon}ka{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}} \right\rbrack}^{\prime}}{{\varepsilon{{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}\left\lbrack {ka{h_{n}^{(1)}\left( {ka} \right)}} \right\rbrack}^{\prime}} - {{h_{n}^{(1)}\left( {ka} \right)}\left\lbrack {\sqrt{\varepsilon}ka{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}} \right\rbrack}^{\prime}}} & (1)\end{matrix}$ $\begin{matrix}{b_{n} = \frac{{{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}\left\lbrack {ka{j_{n}\left( {ka} \right)}} \right\rbrack}^{\prime} - {{j_{n}\left( {ka} \right)}\left\lbrack {\sqrt{\varepsilon}ka{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}} \right\rbrack}^{\prime}}{{{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}\left\lbrack {ka{h_{n}^{(1)}\left( {ka} \right)}} \right\rbrack}^{\prime} - {{h_{n}^{(1)}\left( {ka} \right)}\left\lbrack {\sqrt{\varepsilon}ka{j_{n}\left( {\sqrt{\varepsilon}ka} \right)}} \right\rbrack}^{\prime}}} & (2)\end{matrix}$

Herein, ϵ, k, and α represent the effective permittivity, the wavevector, and the particle radius, respectively, of the microsphere. Also,j_(n) and h_(n) represent the spherical Bessel and Hankel function,respectively.

Further, S₁₁ refers to the angular distribution of scattered light whenilluminated by unpolarized light.

$\begin{matrix} & (3)\end{matrix}$ $\begin{matrix}{S_{11} = {\frac{1}{2}\left( {{❘{\sum\limits_{n}{\frac{{2n} + 1}{n\left( {n + 1} \right)}\left( {{a_{n} \cdot \frac{d^{2}P_{n}}{d\theta^{2}}} + {{b_{n} \cdot \frac{1}{\sin\theta}}\frac{{dP}_{n}}{d\theta}}} \right)}}❘}^{2} + {❘{\sum\limits_{n}{\frac{{2n} + 1}{n\left( {n + 1} \right)}\left( {{{a_{n} \cdot \frac{1}{\sin\theta}}\frac{{dP}_{n}}{d\theta}} + {b_{n} \cdot \frac{d^{2}P_{n}}{d\theta^{2}}}} \right)}}❘}^{2}} \right)}} & \end{matrix}$

Herein, P_(n) is the Legendre polynomial.

From Equation (3), the differential scattering cross section fromunpolarized incident light can be expressed as:

$\begin{matrix}{\frac{dC_{{sca},{tot}}}{d\Omega} = {\frac{1}{k^{2}}{\sum\limits_{i}S_{11,i}}}} & (4)\end{matrix}$

Herein, the sum is over all particles.

Fabrication of Humidity-Responsive Display

Binary displays were created by using particles responsive andunresponsive to humidity for the icon and background, respectively. Thecactus display was fabricated using amorphous titania particles with anaverage diameter of 0.55 μm for the background and 1.26 μm for the icon.The rainy cloud display was fabricated using anatase particles with anaverage diameter of 0.45 μm for the background and amorphous titaniaparticles with an average diameter of 1.45 μm for the icon. Theparticles were dispersed in ethanol at a concentration of 10 mg/ml, andsonicated for over 30 min. For the substrate, a glass slide (2.5×2.5cm²) was cleaned by rinsing for 30 min in acetone followed by 30 min inethanol. A mask with the icon image was attached to one side of theglass slide, and an inverse mask was attached to the other side. Thehumidity-responsive particles were spin-coated onto the masked side at1500 rpm for 30 seconds, and repeated 5 times. Thereafter, theunresponsive particles were spin-coated onto the other side using thesame conditions. The both masks were then removed from the glass slide.

Measurements of Response Time

While a difference in illuminating power of scattered light at a fixedwavelength was monitored, the particles were injected into a largeamount of humid N₂ at variable frequencies (from 3 Hz to 40 Hz) to carryout time-resolved humidity response measurements of the particles. Thehumid flow (90% RH) was generated using a bubbler containing distilledwater, and herein, the N2 gas was injected at about 21 L/min. Themeasurements were carried out at 20% RH dry conditions. The output flowfrom the bubbler passed through an optical chopper (Stanford ResearchSystems, SR540) and targeted to the sample surface. The sample wasirradiated with light output from an optical fiber connected to asupercontinuum laser (NKT Photonics) to monitor changes in scatteringresponse. The laser wavelength was adjusted by an acousto-optic tunablefilter (AOTF) to a value at which the maximum response to changes inhumidity is generated. The scattered signal was collected by a Siphotodiode (Thorlabs, PDA10A2) with an ultrafast rise time (2.3 ns), andread by an oscilloscope (Tektronix, TBS2000).

<Results and Discussion>

Amorphous titania particles with controlled sizes and smooth sphericalgeometries were fabricated using a hydrothermal method with detailsdescribed in the methods section. One key difference between thissynthesis and that of previous reports is that the particles were notannealed but left in an amorphous state. In the present Example, fivedifferent sets labelled in sequence of increasing average diameter fromA to E were prepared. SEM images and the measured size distribution frommore than about 500 particles (FIG. 1A (i) to FIG. 1E (ii)) confirm thepoor spatial order and polydispersed nature of the particles,respectively. To view the scattered colors, the present inventorsprepared a monolayer film of sparsely distributed particles on a glassslide, and observed the forward scattered light in the far-field,generated by a white light source (inset of FIG. 1A (i) to FIG. 1E(ii)). The respective colors of the scattered lights were as follows:Amorphous A [Dry condition: tangerine, Ambient condition: orange],Amorphous B [Dry condition: lilac, Ambient condition: sky blue],Amorphous C [Dry condition: green, Ambient condition: lemon], AmorphousD [Dry condition: yellow beige, Ambient condition: orange], andAmorphous E [Dry condition: brown, Ambient condition: beige]. Eachparticle set generated spatially uniform and distinct colors despite themarginal coverage that changed in dry and ambient environments (i.e.,about 50% RH). Interestingly, when resolved using a 0.9 NA objectivelens in bright field mode, the individual particles appeared colorless,and displayed no color contrast between dry (beige) and ambient (pinkbeige) humidity conditions (FIG. 19 and FIG. 20). These resultshighlight the importance of collective scattering in contrast toindividual scattering for generating distinct humidity-responsivecolors.

To better understand the humidity-responsivity mechanism, anatase TiO₂particles were prepared as Comparative Example by annealing theamorphous particles at 500° C. for 3 hr. As shown in FIGS. 1F (i) and 1F(ii), the anatase particles also exhibit spatial disorder andpolydispersity, but show no color contrast between dry and ambientenvironments (colors of both cases were purple grey). Since physicaldifferences between the amorphous and anatase particles originate fromdifferences in crystalline phase, it can be assumed that the humidityresponsivities depend largely on the microstructure.

This interpretation was supported by a comprehensive analysis of thecrystalline phase, porosity and infrared active modes in both amorphousand anatase particles as shown in FIG. 2A to FIG. 2C. In FIG. 2B andFIG. 2C, each curve is indicated as follows: anatase (

), amorphous A (▪), amorphous B (●), amorphous C (▴), amorphous D (▾),and amorphous E (♦). XRD analysis of the five amorphous particle setsshow no crystalline peaks, in contrast to that of the anatase particles(FIG. 2A), confirming the completely amorphous nature of the particles.It is expected that the amorphous particles are formed through theaggregation of smaller nanocrystallites, leaving nanometer-sized poresat the interstices that establish a dense porous network. BET analysisfrom adsorption isotherms on the five amorphous particle sets indeedconfirm that the particles are highly porous characterized by 1 to 2nm-sized micropores (FIG. 2B and FIG. 2C, Table 1 below) and a smallfraction of 2 to 50 nm-sized mesopores (Table 1). In contrast, theanatase particles show negligible adsorption and porosity. These resultsimply that water molecules can diffuse into the interior of amorphousparticles but not of the anatase particles, rendering only the amorphousparticles responsive to humidity. To confirm this assumption, FT-IRmeasurements were performed on each particle set in dry and ambientconditions. In addition to the anatase particles, FIG. 3A to FIG. 3Cdepict the IR absorption spectra from the D and E amorphous particlesets representing the two extremes in colorimetric range under dry andambient conditions (see FIG. 21A to FIG. 21C for A, B, and C sets). Forthe amorphous microspheres, all infrared active modes including theTi—O/Ti—O—Ti (400-900 cm⁻¹), Ti—O—C (1080 cm⁻¹), —C—O (1235-1457 cm⁻¹),acac ligand (1542, 1575, and 1635 cm⁻¹) and C—H (2860-2936 cm⁻¹)vibrations showed consistent intensities in dry and ambient environmentswith the exception of the O—H vibration (3100-3600 cm⁻¹). The O—Hvibration is present even in dry conditions, but undergoes anenhancement under ambient conditions, verifying the presence of adsorbedwater molecules in the pores. In contrast, the anatase particles,characterized by purely Ti—O—Ti vibrations, showed no difference ininfrared peak intensities under the same conditions since there are nopores into which water molecules can diffuse. This conclusively showsthat the colorimetric humidity responsivity arises from the ability ofthe amorphous particles to accommodate the accumulation of watermolecules in the pores.

Table 1 shows specific surface areas and pore volumes extracted from theBET analysis on N₂ adsorption isotherms of the amorphous A, B, C, D, andE sets and anatase particles. The pore volumes were classified intomicro (<2 nm) and meso-size (from 2 nm to 50 nm) pores.

TABLE 1 S_(BET) Pore volume, V cm³/g) Microsphere (m²/g) V_(Micro)V_(Meso) V_(total) A 356 0.134 0.026 0.160 B 474 0.175 0.035 0.210 C 4910.180 0.022 0.202 D 491 0.177 0.020 0.197 E 380 0.147 0.014 0.161Anatase 2.55 0.44 × 10⁻³ 0.63 × 10⁻² 0.674 × 10⁻²

In the present Example, the relation between pore volume filling andcolor change is described through analytical Mie calculations. FIG. 3Ashows a schematic diagram of the scattering of light by a polydispersedcollection of porous microspheres at 3.3% RH and 97.1% RH, representingthe two extremes in measured RH values. Each microsphere comprises acomplex network of nanocrystallites and pores in which water moleculescan be accommodated. At 3.3% RH and 97.1% RH, the pores were consideredto be completely filled by dry air and water, respectively. Theassumption that water fully occupies the pore volume at 97.1% RH isvalidated later in the discussion of the effective permittivity. For thecalculations, it was assumed that titania particles support negligibleabsorption in the visible range such that the extinction is equivalentto the scattering. Also, in the present Example, the random porousnetwork was modeled as subwavelength spherical inclusions as shown inthe insets of FIG. 4. As the pore sizes determined in FIG. 2C are ordersof magnitude smaller than the visible wavelengths in titania, theextinction can be modeled using effective medium theory. The collectiveextinction spectra represent the sum of individual extinction crosssections from every microsphere in the set. FIG. 5A, FIG. 5B, FIG. 6A,and FIG. 6B show the calculated single-particle extinction crosssections as a function of wavelength and diameter for the D and Emicrospheres, respectively, at 3.3% RH. Overlaid on the calculated mapsare the experimental size distributions, denoted by a square drawn withwhite solid lines whose range is defined by the dotted white box. Byintegrating the product between the size distribution and thesize-dependent cross section over the diameters, the total extinctioncross section can be obtained.

To proceed with the calculations, the permittivity of the titania matrixand mass density of the microspheres were extracted first. Since themicrosphere consists of a dense network of subwavelength pores in atitania matrix, the system can be treated as an effective medium usingthe Maxwell-Garnett formalism. The effective medium approach requires,as inputs, the porosity and the individual permittivities of titania andthe occupied pores. The porosity can be found from the product of thedensity of the microsphere and specific volume (cm³/g) obtained earlierthrough BET analysis (FIG. 2B and FIG. 2C). Although the permittivity ofthe titania and density of the microspheres are unknown, the fiveextinction spectra measured at 3.3% RH conditions present a system ofconstraints that suffice for extracting the two unknowns. In the presentExample, a least-squares algorithm was performed to obtain the twovariables. The extracted density of the microsphere was 2.1 g/cm³ fromwhich the porosity was found to be about 42%. Such a high porosityaccounts for the substantial changes in the effective permittivity underfully dry and saturated humidity conditions. The permittivity of titaniashown in FIG. 22 was on average 29.3% less than that of anatase TiO₂over the visible wavelengths.

The calculated and measured spectra for the amorphous D and E sets at3.3% RH show good agreement, as shown in FIG. 5A, FIG. 5B, FIG. 6A andFIG. 6B, respectively [see FIGS. 23A to 23D, FIGS. 24A to 24D, FIGS. 25Ato 25D, and FIG. 26A to FIG. 26C for the A, B, and C sets. If the valueof C_(ext) becomes large (

10⁷), the color becomes red, and if the value of C_(ext) becomes small (

0.5), the color becomes blue]. As the five measured spectra at 97.1% RHpresent an independent set of constraints, they were used to check thevalidity of the extracted permittivity of titania and density of themicrosphere. In all cases, the calculated results agree well with themeasurements for all particle sets, verifying the accuracy of theextracted parameters, and further confirming the assumption that waterfully occupies the pore volume at 97.1% RH conditions. Although thespectral shape shown in FIG. 5A to FIG. 5D resembles Fabry-Perot (FP)resonances, the spatially dispersed nature of the particles (see FIG. 19and FIG. 20) and the polydispersed size distribution (see FIG. 1A (i) toFIG. 1E (ii)) prove that the particle set cannot constitute awell-defined thin film that supports coherent interference of reflectedwaves which induce FP resonances. Instead, as verified by calculations,the spectra are generated from incoherent scattering in the far-field ofthe particles in different sizes and populations.

The importance of the size distribution in humidity-dependent colorcontrast is illustrated through the amorphous E set, which displays thewidest size distribution among the all sets (full width half-maximum ofabout 0.74 μm). This spread in size suppresses the large spectralvariations observed in the other sets, resulting in a flat curve (FIG.6A to FIG. 6D) that corresponds to a highly desaturated color. FIG. 7Aand FIG. 7B depict the CIE chromaticities from the structural colors forthe amorphous D (FIG. 7A, from ivory to pink) and E (FIG. 7B) sets at3.3% and 97.1% RH conditions. Even though water is present in theinterior of the amorphous E microspheres, as determined earlier by FT-IRmeasurements (FIG. 3A to FIG. 3C), little change in the chromaticitiesbetween the two humidity conditions was observed (both colors of caseswere ivory). This is attributed to the highly desaturated color or flatspectral response, which renders negligible color contrast despiteundergoing spectral changes. On the other hand, the amorphous D set,which exhibits a narrower size distribution (FWHM of about 0.36 μm)displays clearly distinguishable colors between fully dry and humidenvironments. The colorimetric range, therefore, depends strongly on thesize distribution, wherein excessive polydispersity severely limits therange.

Further, in the present Example, the pore volume filling process wasexamined to understand the critical RH at which water fully occupies thepore volume as this defines the colorimetric range and providesinformation on the maximum water uptake amount, normally acquiredthrough gravimetric methods such as dynamic vapor sorption (DVS). In thepresent Example, the normalized extinction cross section was measured atseveral different RH conditions as shown in the top panel of FIG. 8. Ageneral red shift in spectral features is observable for increased RHvalues. To quantify the spectral shift, the peak wavelength at 580 nmwas selected from the extinction spectrum acquired at 3.3% RH as thereference wavelength, λ₀, and its shift for different RH values wasplotted in FIG. 9. The peak shift reaches a constant value after about53% RH, at which point it was presumed that the pore cannot accept morewater vapor as it is completely occupied.

The pore volume filling can be understood through a simple modelschematically shown in FIG. 10. Since water molecules are highly polarand display a strong attraction to one another, it was assumed in thepresent Example that the water vapor forms, on the pore surface, aliquid shell that grows in volume as the humidity increases. Aneffective medium formalism that treats the air-liquid pores ascore-shell inclusions was employed to obtain the effective permittivity.In the present Example, the pore filling fraction was obtained byfinding optimized fits (see the bottom panel of FIG. 8) to the measuredspectra with the least squares algorithm. FIG. 11 plots the calculatedpore filling fraction at different RH conditions. The core-shell modelverifies the earlier assumption that water occupies the full pore volumeabove 50% RH, as the pore filling fraction reaches unity above thisvalue.

Since the pore filling fraction can be converted to the water uptakeamount by taking its product with the microsphere porosity (42%),density of water (1 g/cm³) and specific volume of the microsphere (0.197cm³/g), the pore filling fraction can be directly compared to themeasured water uptake percentage obtained from DVS measurements, asshown in FIG. 11. It is possible to find good qualitative andquantitative agreement between measurement and calculation, whichreinforces the validity of the core-shell model. The close quantitativeagreement also indicates that we can neglect volume swelling of themicrosphere upon water uptake as an additional mechanism for colorcontrast since this would alter the specific volume of the microspheres,resulting in larger deviations between the calculation and measurement.Further, in the present Example, the close agreement suggests that wateruptake measurements on porous particles, which are critical fordetermining their hydroscopicity, can be effectively determined throughoptical measurements shown here with only a fraction of sample amountand time used in gravimetric measurements.

The reversibility of the pore volume filling process was studied byrepeatedly subjecting the microspheres to 3.3 and 97.1% RH cycles. FIG.12 shows Δλ₀ under multiple cycles. Although the chromaticity appearsrelatively consistent through the multiple cycles as seen in the insetof FIG. 12 (mostly ivory at RH 3.3% and mostly pink at RH 97.1%), thepeak shift values at 3.3% and 97.1% RH steadily increase from 0 nm and67 nm to 10 nm and 72 nm at the 10^(th) cycle, respectively. The factthat the increase in peak shift for the dry condition is more drasticthan that for the fully humid condition suggests that water does notfully desorb in dry conditions and remains in the pore presumably due tochanges to the surface chemistry of the pores.

One consequence of the large size and refractive index of themicrospheres is that their scattering is dominated by forwardscattering. FIG. 13A and FIG. 13B depicts the calculated totaldifferential scattering cross section from the microspheres as afunction of angle and wavelength in dry and ambient conditions [If thevalue of dC_(sca)/dΩ becomes large (

10⁷), the color becomes red, and if the value of dC_(sca)/dΩ becomessmall (

10⁵), the color becomes blue]. It can be seen that for both environmentsthe scattering is peaked at 0° (red), corresponding to the forwarddirection (Blue toward −150° C. or 150° C.). The calculations wasqualitatively corroborated by evaluating the colors in the two humidityconditions under different illumination angles as shown in FIG. 14.Here, the sample and camera are fixed in position while the illuminationangle varies. FIGS. 15A-15R depict the color variation from themicrospheres at several different illumination angles up to 26.6°. Itcan be seen that the scattered color appears strongest at 0°, aspredicted by calculations (Dry condition: beige/Ambient condition:pink). At larger angles, the dry and ambient microspheres lose theircolor vibrancy as scattering weakens to the extent that the two humidityenvironments induce similar color impressions.

A primary strength offered by disordered structural colors is thatcolor-generating devices can be fabricated easily and cheaply usingsolution-based methods as opposed to the case of ordered structuralcolors where complex lithographic methods are required. By spin coatingtitania particles onto transparent substrates, humidity-responsivecolorimetric designs can be easily realized due to the lack ofrequirement on spatial order. Two Examples are illustrated in FIG. 16A(i) to FIG. 16B (iii), FIG. 17, and FIGS. 18A to 18H, where thebackground and simple icon image were prepared by spin coatingseparately particles unresponsive and responsive to the humidity,respectively. FIGS. 16A (i) to 16A (iii) depict a display that activatesa green cactus when dry (Green is not active in ambient condition) andFIGS. 16B (i) to 16B (iii) depict a separate display that activates ablue rainy cloud when humid (Blue is not active in dry condition).

Optical microscopy analysis shows that the particle coverage onlyamounts to one to two monolayers. To evaluate the scattering behavior ofthe particles at different particle densities, the display was rotatedunder an aligned illumination and detection pathway, as shown in FIG.17. At high rotation angles, the density of microspheres presentedbefore the beam path increases, causing multiple scattering and colordegradation. FIGS. 18A to 18H illustrate the rainy cloud icon rotated upto 60° with respect to the initial sample plane. It can be seen that fordry (Blue of the rainy cloud was not activated) and ambient (Blue of therainy cloud was activated) conditions, the icon and background colorcontrast gradually decreases at larger rotation angles due to theapparent increase in particle density. This further illustrates that theoptimum scattering contrast occurs at a microsphere coverage of only oneto two monolayers, which signifies the need for only a marginal amountof material to produce saturated structural colors.

An additional key advantage supplied by the monolayer coverage is thefast response rates to humidity changes. The monolayer coverage impliesthat the particles are immediately exposed to water molecules in humidenvironments in contrast to the case where they are buried insideseveral layers of particles. Therefore, treatment of water moleculetransport through several layers of particles can be excluded, andinstead, the diffusion of water molecules into the porous network of anisolated sphere can be considered. The solution to Fick's second law ofdiffusion in spherical coordinates, subject to the infinite bathboundary conditions is presented. Although the minimum time required forthe water molecules to distribute evenly across the entire particleideally represents the response time, even when the particle centerreaches 90% of the outer concentration, the effective permittivity islargely unaffected because the mass ratio relative to the equilibriummass remains near unity (97%). Therefore, in the present Example, theminimum time for the concentration at the particle center to reach 90%of the input concentration was calculated as the response time. Previousstudies on the diffusion and transport of molecules in porous networkshave shown that the pore size governs the diffusion dynamics. Forexperimental pore sizes of from 1 nm to 2 nm, it is known thatintracrystalline dynamics dominate, yielding diffusion constants in therange of from D^(˜)10⁻⁷ m²/s to 10⁻¹² m²/s. This range corresponds tothe response time of from 0.37 μs to 37.2 ms for average diameter of 0.7μm and from 1.49 μs to 149 ms for average diameter of 1.4 μm.

In the present Example, an ultrafast response rate of the microspherewas verified by measuring time-resolved humidity response measurements.FIG. 27 is a schematic diagram showing a setup for the present test.Pulses of humid N₂ flow generated by periodically modulating the outputflow from a bubbler were delivered onto a monolayer of amorphous Bparticles (average diameter of ^(˜)1 μm) with RH values cycled between˜20% and ˜90%. In the present Example, to probe the temporal response ofthe particles, the sample was illuminated with a supercontinuum lasertuned to a wavelength of 690 nm that was verified as generating thelargest response to humidity changes. Temporal changes in the forwardscattered signal were then detected by a Si photodiode with a ^(˜)2 nsrise time and monitored by an oscilloscope.

FIG. 28 shows the response times of the particle set subjected todifferent humidity modulation frequencies of from 3 Hz to 40 Hz. Therise (recovery) time is defined as the time interval for a signal toincrease (decrease) from 10 (90)% to 90 (10)% of the saturated voltage.As can be seen from FIG. 29, the asymmetric S-shaped fit of the signalat 3 Hz under control conditions indicates the rise time and therecovery time of 18 ms and 33 ms, respectively. This value is similar tothe conventional fastest response time (^(˜)30 ms) of a grapheneoxide-based electrical humidity sensor and two to three orders ofmagnitude faster than an electrical sensor manufactured using a TiO₂thin film. Since a cycle (25 ms) of the particle set at 40 Hz is shorterthan the sum of the rise and recovery times, it can be seen that even ifthe response signal is not saturated any more, the response is fastenough to trace the modulation. By inputting the rise time of 18 ms intothe above-described simple diffusion model, the diffusion coefficient ofwater molecules adsorbed into the porous titania microsphere can beestimated as D^(˜)4.5×10⁻¹² m²/s which is suitable for the particle witha pore size of from 1 nm to 2 nm.

<Conclusion>

In summary, in the present Example, the use of amorphous titaniamicrospheres in humidity-responsive colorimetric sensors wasdemonstrated. The amorphous titania microspheres exhibit negligiblespatial order, and yet generate colorful scattered light that respondsto different humidity conditions. Each microsphere individually scatterslight that appears colorless, due to its noisy spectrum; however, apolydispersed collection of such microspheres scatters saturatedcolorful light due to the washing out of high frequency spectral noiseand the exposing of smooth-varying spectral variations by the additivecontributions of scattering from multiple particles of different sizes.In the present Example, through BET analysis, FT-IR, and opticalcalculations using an effective medium approach, it was conformed thatthe structural color contrast mechanism at different RH conditionsoriginates from water molecule adsorbing and condensing in the highlyporous titania network of the amorphous microspheres, whichsignificantly changes the effective permittivity of the microspheres.The diffusion of the water molecules in the porous network isessentially fast and exhibits a response time of from 18 ms to 33 ms.The response time is similar to that of a fastest electrical humiditysensor based on graphene oxide and is the fastest among the colorimetricsensors. Using extracted parameters, the water uptake was evaluated froma purely optical analysis using only a fraction of sample amount andtime required by gravimetric analysis. This opens up promising pathwaysfor using optical measurements on dielectric particles as an alternativeand efficient approach for acquiring the water uptake. For cases wherethe material density and permittivity are known, this method alsoenables the porosity to be obtained. Furthermore, due to the lack ofconstraints on spatial order, the microspheres can be easily applied tocolorimetric humidity-sensitive displays via spin coating. To this end,simple and ultrafast humidity-responsive icons were described andoptimized both in speed and signal at only a monolayer coverage. Theseresults point to practical and useful opportunities for exploitingdisordered structural colors in ultrafast humidity sensitiveapplications.

The above description of the present disclosure is provided for thepurpose of illustration, and it would be understood by a person withordinary skill in the art that various changes and modifications may bemade without changing technical conception and essential features of thepresent disclosure. Thus, it is clear that the above-described examplesare illustrative in all aspects and do not limit the present disclosure.For example, each component described to be of a single type can beimplemented in a distributed manner. Likewise, components described tobe distributed can be implemented in a combined manner.

The scope of the present disclosure is defined by the following claimsrather than by the detailed description of the embodiment. It shall beunderstood that all modifications and embodiments conceived from themeaning and scope of the claims and their equivalents are included inthe scope of the present disclosure.

We claim:
 1. A method of preparing a colorimetric humidity sensor,comprising: synthesizing amorphous microspheres by a non-aqueoussolvothermal method to prepare humidity-responsive particles; andcoating the humidity-responsive particles in a monolayer on atransparent substrate to form a colorimetric member wherein thecolorimetric humidity sensor comprises the colorimetric member includinghumidity-responsive particles configured in a disordered monolayerarrangement on a substrate.
 2. The method of claim 1, wherein thehumidity-responsive particles are amorphous, porous, and polydispersedmicrospheres, and wherein the colorimetric humidity sensor indicates ascattered color change according to humidity upon light irradiation. 3.The method of claim 1, wherein the humidity-responsive particles includeat least one oxide, wherein the at least one oxide includes at least oneselected from SiO₂, TiO₂, BaTiO₃, ZnO, Ta₂O₃, Nb₂O₃, CaO, Li₂O, SnO₂,Sb₂O₃, Sb₂O₄, As₂O₃, SrTiO₃, PbTiO₃, or CaTiO₃.
 4. The method of claim1, wherein an average diameter of the humidity-responsive particles isfrom 0.05 μm to 10 μm.
 5. The method of claim 1, wherein the amorphousmicrospheres are synthesized to have different average diameters,respectively.
 6. The method of claim 1, wherein the colorimetric memberindicates a different color according to an average diameter of thehumidity-responsive particles.
 7. The method of claim 1, furthercomprising: coating humidity-unresponsive particles in a monolayer onthe substrate in a region of the substrate where the colorimetric memberis not formed.
 8. The method of claim 7, wherein thehumidity-unresponsive particles are nonporous crystalline particles. 9.The method of claim 1, wherein the colorimetric humidity sensor furthercomprises a second colorimetric member including secondhumidity-responsive particles configured in a disordered monolayerarrangement, wherein an average diameter of the secondhumidity-responsive particles is different from that of thehumidity-responsive particles.
 10. The method of claim 1, whereinmoisture is adsorbed in pores of the humidity-responsive particles. 11.The method of claim 10, wherein the average pore diameter of thehumidity responsive particles is from 1 nm to 60 nm.
 12. The method ofclaim 1, wherein the colorimetric humidity sensor indicates a change insaturation of the color according to an angle of light irradiation. 13.The method of claim 1, wherein the color change of the colorimetrichumidity sensor upon light irradiation is measured by an opticalmeasuring instrument using at least one selected from a photodiode, acharge coupled device (CCD), and a complementary metal oxidesemiconductor (CMOS).
 14. The method of claim 1, wherein the colorchange is observed within a narrow range of polydispersity, which is thefull width half maximum less than 0.36 μm.
 15. The method of claim 1,wherein the color change is distinguishable in a wide range ofpolydispersity, which is the full width half maximum exceeded 0.74 μm.